Here in both cases the same amount of heat is to be supplied, i.e.,

H1 = H2

\(\frac{V^2}{R_1}t_1=\frac{V^2}{R_2}t_2\)      \(\left[\because H=I^2Rt\right]\)

Or, \(\frac{t_1}{R_1}=\frac{t_2}{R_2}\)

Since t1 is greater than t2 so, R2 is less than R1

As we know that, the resistance is directly proportional to the length, the length should be decreased of the heating element in which water boils in 10 minutes.